A coil is an electrotechnical structure that is formed by winding an electrically conductive conductor, i.e. winding wire, in turns. Coils can be employed in connection with a plurality of electromagnetic and electromechanical devices. Examples of these devices include a choke, a transformer, a motor and a generator, all of which can include one or more coils.
A winding wire can be of any electrically conductive material, but various metal materials, such as copper and aluminium or alloys thereof, can be used, depending on the application for which the coil is intended. On the surface of the winding wire, there can be an insulating layer, such as a varnish, if the turns in the coil come into contact with one another. The cross-section of the winding wire can be round or rectangular, for example. FIG. 1 shows the cross-section of a rectangular winding wire 10.
When the coil is in use, an electric current passes therethrough and causes losses, which in turn heat the coil. In general, the coil can be cooled by air cooling, whereby excessive heat is conducted and radiates from the coil surfaces to the space surrounding the coil. Air cooling can be enhanced, when necessary, by a fan. However, air cooling of this kind may not provide a sufficient cooling effect in all conditions and applications. In addition, heat conducted and radiating from the coil to the space surrounding the coil can be harmful to other devices or structures in the vicinity of the coil, which may further increase the necessary cooling effect.
U.S. Pat. No. 6,741,152 discloses cooling a coil using cooling channels or cooling tubes, in which a coolant flows. The cooling channels or cooling tubes are placed inside coil conductors, which have at least two profiled conductor segments, or inside stranded conductors. Even though it can be possible to cool the coil by the disclosed solution, the solution does not necessarily reduce heat conduction and radiation to the space surrounding the coil.